Manufacture of hyposulphites



o. NITZSCHKE El AL MANUFACTURE OF HYPOSULPHITES Fi1ed Sept 15, 1938 Oswn Nzftschke Ml/mch, Kopsch INVENTORS BY H i; 0L THE/R ATTORNEYS Patented Mar. 12, 194i) 1 UNITED: STATES PATENT.

I 2.193.323 MANUFACTURE or osunrm'rns the-Main, Germany Application September 13, 1938, Serial No. 229,706

Germany May 10, 1985 4 Claim.

This invention relates to the manufacture of water-soluble hyposulphites, more particularly to the manufacture of sodium' hyposulphite by cathodic reduction of bisulphite-containing solutions.

It is known to prepare hyposulphites, for instance, alkalior alkaline-earth metal hyposulphltes by electrolyzing bisulphite-containing solutions as catholyte in an electrolyzlng cell which is divided into an anode and cathode chamber by means oi a diaphragm.

Since hyposulphite solutions tend to a quick decomposition this process causes many difflculties, so that up to the present day no technical manufacture or hyposulphites by cathodic reduction of bisulphites is realized. Though many in vestigators have studied the conditions which are to be observed for obtaining concentrated solutions of hy'posulphites with satisfactory current 'and bisulphite yields, no commercial process has current yields did not exceed 30%. .Anotherprocess'consist's in. performing the electrolysis at low temperatures, preferably at 0-5 6., at any rate not essentially above 15 C. To prevent the hyposulphite-ions to,migrate to the anode, a part of the anolyte is caused to pass through the diaphragm into the cathode chamber. In

' this way concentrated hyposulphite solutions may be produced, butthe current yields after having 0 attained a concentration of 14%-15% of sodium hyposulphite likewise amount only to 30%. Moreover the necessary strong cooling aggravates the technical performance of the process. Finally a process is based upon rapidly stirring the catholyte. This process permits the production 01 concentrated sodium hyposulphite whereby the current yield may even rise to 50%. But it has proved, that this rate of current yield does not satisfy, because in the manufacture of hyposulphite solutions low current yields are connected with the formation of byproducts which remain dissolved in the catholyte and exordinarily disturb the working-up of these solutions.

k In accordance with the present invention hysolutions posulphites are produced in a technically simple cell and by a simple method attaining high current concentrations, for instance, -95%. The process comprises subjecting alkali metalor .zinc bisulphite-containing solutions with a pH or 4.5-6.0 .as catholyte to electrolysisunder rapid stirring; at current concentrations between 2 and 50 amperes per com. catholyte, at temperatures between 0 and 50 (3., preferably at 25-40 C. and cathodic current densities between 0.5 and 50, preferably at 2-20 amperes per 100 qcm. surface of the cathode, wherebySOz is introduced into the catholyte so that the said acidity is maintained during the whole process of electrol-- ysis, while simultaneously an aqueous solution of an alkaline reacting, alkali metal compound is introduced into the anode chamber and a substantial part of the anolyte is forced through the diaphragm into the cathode chamber.

The electrolyzing cell may be of optional construction so far as it permits to apply current concentrations up to 50 amperes per 100 com. catholyte, that is to say: the cathode chamber, the quantity of catholyte respectively, must be'small in proportion to the charge of the cell with current. In the annexed drawing a cell of tubular construction is shown by way of example. In this drawing (I) represents a cylindrical vesselywhich may consist of lead or silver, which is connected with the negative pole of the source of current. Within this vessel a clay tube (2), which is provided with a bottom, is arranged as a diaphragm in a distance of about 8 mm. from the cylindrical side of the vessel. Inside of the diaphragm a tube (3) is placed, which is connected with the' positive pole of the source of current; it may be oi'lead, chromium-nickel-steel or graphite corresponding to the anolyte employed. The cell is closely shut by the'rings (4) and (5) of rubber. By pipes (6) and (1) the solutions, which form orflll up the anolyte or the catholyte are introduced. By pipe (8) the developed oxygen at the anode and, in a given case, also a part of the anolyte leave the cell. Pipe (9) represents the overflow pipe for the catholyte. Around the vessel (I) there is arranged a cooling jacket (I0). At the bottom of the vessel (i) a perforated ring (I I) is provided, by which S02 may be finely divided which is introduced by pipe (I!) If this cell is charged with current, for instance,

, so that a current density of 10 amperes per 100 qcm. is on the cathode, the current concentration at a distance of 8 mm. between diaphragm (,2) and vessel (i) amounts to about 12 amperes per 100 com. of catholyte. When using higher or current Concentration may be readily shifted to higher or lower values.

According: to the same principle cells of filterpresslike construction or any other form may be constructed, whereby diaphragms of porous rub- ,ber or any other flexible material may be used instead of claydiaphragms. Also the anode may be constructed in form of a vessel or a chest provided with a cooler. By pipe- IS) the cooling water is introduced leaving the apparatus by pipe (ll) I On the other hand, in certain cases, cooling of the cathode is superfluous, if the anode is cooled.

'These. and many other variations are possible for the construction of a cell. suitable for the present process. v

The working of the cell may be as well periodically as continuously, that is to say, in the first case the cell is filled with the solutions, provided as anolyte and as catholyte, then electrolysis is eflected; finally both, catholyte and anolyte are removed and replaced by fresh solutions, or the catholyte alone is periodically replaced while the anolyte is continuously replaced; in the other case the corresponding solutions flow continuously as well through the cathode chamber as through. the anode chamber while electrolysis is effected. As catholyte solutions containing bisulphites of alkali metals or of zinc, but also solutions of neutral salts of these metals to which bisulphites are added or in which bisulphites are formed by introducing S02, are used. The acidity, which is measured by means of a glass-electrode is to be kept between pH=4.5 and pH=6.0-; preferably between pH-= 5.0 and 5.5.- This acidity corresponds to a mixture of bisulphite with -50% of sulphite. Up to the present day it was not known,

that the formation of hyposulphite depends so very much on the acidity of the catholyte. Hitherto a mixture of 95% of sodium bisulphite and of .sodium sulphite, the pH of which is about 5.0, and a mixture of 95% of sulphite and 5% of bisulphite, the pH of which is more than 7.0, have been suggested as being equivalent. However, it has been found that the current yields in both cases are quite different. Under equal conditions there are obtained in the former case more than90%, in the latter case scarcely 20%. It is an essential characteristic of this invention that the acidity for the whole duration of the electrolysis is maintained between a pH of 4.5 and 6.0. Stirring of the catholyte may be effected in any convenient manner. A suitable method is by pumping the catholyte with great speed through the cathode chamber along the cathodes. For this purpose a pump is arranged at the outside of the cell (not shown in the drawing). The

liquid circulates through pipe (1) into the cathode chamber, leaving the latter by pipe (9) passing a small intermediate vessel where gases may be given on. and returns into the pump. S02 may be introduced into the catholyte not only by means .of the perforatedring (I l) but also from outside the cell before or behind the pump in a mixing vessel.

This

tity of catholyte is obtained and a too sudden absorption in the neighbourhood of the distributor is prevented. In this manner local superacidification, which would reduce the current yield is easily avoided.

A further advantage of stirring with gases is the diminution of the quantity of liquid, which is present in the cathode chamber. Thus, the current concentration may be increased to high values.

The cathodic current density may vary between wide limits, but is not allowed to surpass a certain value, as otherwise formation of hydrogen and secondary reactions would take place. This boundary current density differs according to the form of the surface of the cathode and to the intensity of stirring. In general, however, 50 amperes per 100 qcm.should not be surpassed. Cathodes the effective surface of which consists in an arrangement of thin wires were found to be particularly advantageous. Thin wires in this case, are wires of a. diameter of about 0.1-2 mm., preferably of 0.1-0.5 mm. The wires are arranged in such a manner, that intervals are formed between the single wires, so that the catholyte passes through and thus comes into intimate contact with the surface of the wires. The intervals betweenth wires or, if the wires are netlike arranged, the meshwidth, are about 0.1-0.5

mm. In the drawing, the wire-net is represented by the dotted line (I5). It is soldered to the vessel (I).

The temperature of the catholyte is not re,- stricted to temperatures below 15 0., but may rise up to C. Preferably the process may be carried out at 25-40 C. Since no cooling or only cooling with simple means is necessary, this process is cheaper than the usual ones. Also, on account of the lower viscosity of the solution stirring or moving of the electrolyte is more effective.

Alkaline reacting alkali metal compounds suitable for effecting the process are solutions of alkali metal hydroxides or carbonates. It may, however, be advantageous to use a solution of ,neutral alkali metalsulphite.

' also serves,for facilitating the transport. In this manner the diffusion of hyposulphite anions to the-anolyte is avoided, whereby an important increase of the yield is attained.

The invention is especially illustrated by the following examples without being restricted thereto.

Example 1 In a tubular-cell as described above and being provided with an anode of chromium-nickel-steel and a cathode of silverplate, a 23% sodium-bisulphite solution as catholyte is electrolyzed while using a cathodic current density of 6 amperes per 100 qcm. and a current concentration of 10 amperes per 100 com. of catholyte. By introducing S02 into the catholyte the pH is kept at 5.2-5.5 for the whole duration of the electrolysis. Stirring of the catholyte is effected by rapid circulation by means of a pump. The temperature is chamber. is'so adjusted, that it is higher than I the hydrostatic pressure in the cathode chamber 7 so that an essential part of the anolyte is pressed through the diaphragm into the cathodic chamher. In consequence of this transport'the volume of the catholyte increases for about 50% until a 14-15% hyposulphite solution is attained. If at this 'state electrolysis isinterrupted, the medium current yield calculated on hyposulphite'is 73%.

It now between vessel (i) and diaphragrn (2') a silver wire net of a meshwidth of 2 mm. and a wire-thickness of 0.25 mm. is arranged in such a manner that the net at the upper and lower end is soldered to the vessel the current'yield rises to 78%. V

A similar-sheet is obtained if in the original arrangement with the silver plate cathode stirring is not efi'ected by pumping, but bypassing through an inert gas, as CO2, N2, H2. The current yield in this case amounts to 80%.

If finally the use of gasstirring is combined with the employment of the wire-net-cathode as above described, the current yieldw rises to 90%.

- Example 2 In an electrolytic cell as described above and in the form of a filter press cell, which is divided ous bisulphitecontaming solution at a current concentration between about 2 and about 50 aminto anode andcathode chambers by means of a porous rubber plate as the diaphragm, in which -cell the anode consists of alead plate and the cathode is a silver wire net, a 16% sodium bisulphite solution as the catholyte is subjected to electrolysis at C. while using a 17% sodium sulphite solution as the anolyte, a cathodic current density of 14 amperes per square decimeter and a current concentration of 8 amperes per 100 cos. of catholyte.

passes through the anode chamber and the oatholyte passes through the cathode chamber. During the electrolysis carbon dioxide, hydrogen or nitrogen is introduced at the lower part of the cathode chamber for stirring the catholyte. By

adding SOzto the said stirring gases the necessary' S02 is conducted to the catholyte while simultaneously establishing the desired degree of acidity. Now, by causing theacidity to correspond to different pH-values by the corresponding addition of S02, and takingcare that always yield. The pH-values in this case relate to. values measured by meansof the glass electrode at 20" C. According to the intensity of stirring, the values differ somewhat in such a manner that they are more favorable with more vigorous stirring and lessfavorable with less, good stirring.

The bath tension differs according to the various degrees of acidity. It is thelower the more acid is the catholyte. 4

If the temperature of the catholyte is fixed to 43 C. under otherwisethe same conditions as in- 15% hyposulphite solution with current Anolyte and catholyte pass through the cell continuously, that is the anolyte yield, at a pH-value of 5.6 a 16% solution with i current yield, at a pH-value of 4.85 a 15% solution with 85% currentyield. -The bath tension is about 10-15% lower than at 15 C.

All conditions not specifically mentioned in the foregoing example are identical with those given in Example 1, especially a substantial part of the anolyte is forced by hydrostatic pressure through the diaphragm into the cathode chamber.

This application is a continuation in part to application Ser. No. 77,641, filed May 2, 1930.

i We claim:

laProcess for preparing water soluble hypo-' sulphites which comprises electrolyzing an aqueous bisulphite-containing solution at a current 'concentration between about 2 and about 50 amcurrent density between about 0.5 and about 50 amperes per qcm. cathode surface, while maintaining the acidity of the catholyte during the electrolysis, at a pH-of between about 4.5. and about 6.0 by introducing sulphur dioxide into the catholyte, and forcing a substantial part of the anolyte through the diaphragm into the cathode chamber, said anolyte consisting of an alkaline reacting aqueous solution 01' an alkali metal coinpound,while rapidly stirring the catholyte.

2. Process for preparing water soluble hyposulphites which comprises electrolyzing anaqueanolyte consisting of an alkaline reactingaaueous solution ofan alkali metal compound.

3.'Process for preparing water soluble hypo- .sulphites which comprises electrolyzing an aqueous bisulphite-containing solution at a current concentration between about 2 and about 50' amperes per 100 "com. catholyte, at a temperature between about 0 andabout 50 (2., at cathodic current density between about 0.5 and about 50 amperes per 100 qcm. cathode surface, while maintaining the acidity of the tholyte during the electrolysis at a pH of betweg about-6.0 by introducing sulphur dioxide into the catholyte, and forcing a substantial part of the about 4.5 and anolyte through the diaphragmin the cathode chamber, said anolyte consisting 0 'an alkaline reacting aqueous solution of an alkali metal compound, while rapidly stirring the catholyte, the cathodes consisting of thin wires having a diameter ofabout 0.1-2 mm. being arranged at intervals of about 0.1-0.5 mm.

a 4. Process for preparing water soluble hyposulphites which comprises electrolyzing an aqueous bisulphite-containing solution at a current concentration between about 2 and about 50 amperes per 100 com. catholyte, at a'temperature maintaining the acidity of the catholyte during the electrolysis at a pH of between about 4.5 and about 6.0 by introducing sulphur dioxide into the I 4- 9,198,828 catholyte, stirring the catholyte by means of a cathodes consisting of thin wires having a diamgas inert to the reagents of the process,- and eter 0! about 0.1-2 mm. being arranged at interforcing a substantial part of thev anolyte through the diaphragm into the cathode chamber, said anolyte consisting of analkaline reacting aque- OSWIN NITZSCHKE, 6 ous solution of an alkali metal compound, the ULRIOEKOPSCH.

vals of about 0.1-0.5 mm. 

